Conventional locks employ a key-operated plug rotatably mounted in a cylindrical shell. Absent the key, the combinating tumblers, which may be disks or pins, are biased to block rotation of the plug With disk tumblers, absent the key, the combinating disks extend into a longitudinal groove in the shell and prevent rotation of the plug. When the combinating tumblers read a properly bitted key inserted in the keyway they are set to the shear position and the plug is free to rotate: the lock is unlocked. Another groove may be provided to permit withdrawal of the key in the unlocked position. Each groove may be paired with a companion, diametrically opposed groove to allow disk tumblers to shift about as a key is inserted and withdrawn.
It is often necessary to replace the plug upon change of employee or tenant to prevent subsequent use of their key to gain access after they have left. Also equipment such as desks, filing cabinets, doors and the like are often supplied without plugs so that the locks in a particular set of desks, doors or other equipment can be actuated by common key.
In both cases, the replacement and installation, it is often necessary to use skilled labor. In some cases the plug is secured by means of a cam or other actuator on the back of the shell; in other cases a slidable member in the plug engages the shell and prevents withdrawal unless the member is depressed by a tool inserted through a hole in the shell. In both arrangements withdrawal is not possible unless the rear of the lock is accessible, which can be troublesome if the keys to a locked desk or filing cabinet are lost.